Richard Jenny – AGVISE Agronomist
Recently during a presentation on the importance of long term P & K research to improve fertility management, the person next to me said “why don’t we just use crop removal (CR) rates for P & K fertilizer recommendations?” I was a little surprised by this comment, and replied to him that if you only use CR rates, then most of a field would either be over or under fertilized. When soil samples test in the high or very high range for P & K, then applying the CR rate would result in an over-application. If soils testing low or very low receive the CR rate then P & K would be under fertilized and reduce yield. The only situation where the CR rates would be about right is when the soil test for P & K are in the medium range.
Another factor to consider is that a tremendous amount of time, labor, energy and money have gone into many decades of research to help develop and update fertilizer recommendations so we apply only the P & K fertilizer that is needed. When CR rates are applied on high testing soil, the result is costly over-application and it also increases the risk of off-site movement, especially phosphorus. Phosphorus movement downstream into ditches, streams, rivers and lakes contributes to water pollution. In the case of low testing soils, using the CR rate, will result in under-application which would likely reduce crop yields and reduce producer profitability. The only solution to this situation is to do intensive soil sampling (grids or zones) and use research from your region to manage P & K fertility to maximize yields, profitability and protect the environment.
In many states, where producers receive federal or state conservation incentive payments, they are usually required to follow university fertilizer guidelines. These guidelines can come from the state where they live or a neighboring state. These University guidelines prevent gross over or under application of P & K because they are based on years of regional research. Using only crop removal rates to make P & K fertilizer guidelines is a bad idea.
Below are figures of P & K fertilizer guidelines comparing the crop removal (CR) rate to University guideline (1-year sufficiency), AGVISE band guideline and the AGVISE build guideline. The AGVISE build guideline is designed to bring the soil test P & K test level to the High soil test range over a 5-7 years and then maintain them in the High soil test range. The build guideline is often used when growing high value crops. The AGVISE Band guidelines will slowly build P & K test levels to the medium test range over many years and is used primarily in small grain areas. You can see that at very low and low P & K test levels, the crop removal rate (CR) is lower than any of the fertilizer guidelines, including the university guidelines. This means if you only apply crop removal rates to low testing soils, yield will likely be lost due to lack of P & K fertility. At H and VH soil test levels, applying crop removal rates results in over application, reduces grower profits and will put the environment at risk.